Process for producing metal foil

ABSTRACT

The present invention relates to a process for electrolytically producing metal foil which is diminished in pinhole defects and has a uniform thickness. The metal foil is produced by passing an electric current between a cylindrical cathode immersed in an electrolytic solution and an anode opposed to the cathode, continuously electrodepositing a metal layer on the surface of the cathode while rotating the cathode and thereafter peeling the metal layer off. An auxiliary anode capable of adjusting the current density when electrodeposition is started is disposed at a position downstream from the anode with respect to the direction of flow of the electrolytic solution. The auxiliary anode is an electrode having a coating layer comprising an electrode active substance and formed over an electrically conductive metal substrate, with an intermediate layer of tantalum or a tantalum alloy formed between the coating layer and the substrate.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a process for producing metalfoil, and more particularly to a process for continuously producingmetal foil by electrodepositing copper, nickel or like metal foil on arotating cylindrical cathode and thereafter peeling the foil off, theprocess using an auxiliary anode in addition to the main anode foradjusting the current density when starting the electrodeposition so asto produce foil having a uniform quality and free from pinhole defects.

[0002]FIG. 2 shows a typical process for producing metal foil, i.e., aconventional process wherein an electrolytic reaction is conducted forproducing copper foil for use in printed circuit boards or the like. Theprocess uses an electrolytic vessel provided with a cylindrical cathode1 rotatable in the direction of an arrow A, and an anode 2 having acurved surface opposed to the cathode. An electrolytic solution isforced against and supplied to the cathode from a slit 4 in the centrallower portion of the anode and is thereby passed through a clearance 5between the cathode and the anode to conduct electrolysis andelectrodeposit metallic copper foil of predetermined thickness on thesurface of the cathode. The copper foil is peeled off and wound up by awinder 6 for continuous production.

[0003] The anode for use in such an electrolytic bath is conventionallymade from lead or lead alloy, whereas lead has the problem of becomingconsumed relatively rapidly to contaminate the electrolytic solution anddegrade the product with dissolved lead. Accordingly, various insolubleanodes are proposed recently which comprise an anode material and anelectrode-active coating layer formed on the material and containing anoble metal oxide.

[0004] Although the problem of contaminating the electrolytic solutionand degrading the product has been ameliorated to some extent by the useof this type of insoluble anode, it is required to provide copper foilof ever decreased thickness with an increase in the complexity ofprinted circuit boards in recent years. Production of thinner copperfoil increases the possibility of developing pinhole defects, so thatvarious means have been proposed for controlling the current density forthe initial stage of electrodeposition by providing an auxiliary anodein addition to the main foil producing anode [e.g., JP-A No.10-18076(1998)].

[0005] When the auxiliary anode is to be used in existing copper foilproduction apparatus, the location where this anode is to be installedis greatly limited, with the result that the insoluble anode becomesinoperative for electrolysis or locally inactivated within a shortperiod of time, for example, because of a relatively small electrodearea and a relatively high current density. When a conventionalinsoluble electrode is used as an auxiliary anode, it becomeselectrolytically inoperative within a short period of time. Therefore itis frequently replaced, because local inactivation results in unevendeposition on the cathode, leading to the production of faulty product.The use of the expensive noble metal as an electrode active substancefor conventional auxiliary anode is not advantageous economically.

SUMMARY OF THE INVENTION

[0006] It is required that the auxiliary anode of the copper foilproduction apparatus should be operated with a pulse current or at ahigh current density of at least 50 A/dm² in view of the purpose of use.The conventional electrode containing a platinum group metal oxidebecomes electrolytically inoperative or locally inactivated within ashort period of time if operated under such a condition. We haveconducted extensive research in an attempt to overcome this problem andfound that an auxiliary anode having a tantalum intermediate layerformed between a titanium substrate and a coating layer of an electrodeactive substance containing a platinum group metal oxide is exceedinglysuperior in function and durability to electrodes comprising a titaniumsubstrate and the same coating layer as above formed directly on thesubstrate. Thus the present invention has been accomplished.

[0007] In producing metal foil by passing an electric current between acylindrical cathode immersed in an electrolytic solution and an anodeopposed to the cathode, continuously electrodepositing a metal layer ona surface of the cathode while rotating the cathode and thereafterpeeling the metal layer off, the present invention provides a processfor producing metal foil characterized in that an auxiliary anodecapable of adjusting the current density when electrodeposition isstarted is disposed at one side of the cathode where an unelectrolyzedportion thereof is brought into the electrolytic solution, at a positiondownstream from the anode with respect to the direction of flow of theelectrolytic solution, the auxiliary anode being an electrode having acoating layer comprising an electrode active substance containing aplatinum group metal or a platinum group metal oxide, or a mixture of anoxide of a valve metal and a platinum group metal or a platinum groupmetal oxide and formed over an electrically conductive metal substratecomprising titanium or a titanium alloy, with an intermediate layer oftantalum or a tantalum alloy formed between the coating layer and thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a view in vertical longitudinal section schematicallyshowing an embodiment including an auxiliary anode; and

[0009]FIG. 2 is a view in vertical longitudinal section schematicallyshowing a conventional apparatus for electrolytically producing copperfoil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The auxiliary anode for use in the present invention has anelectrically conductive metal substrate which is made from metallictitanium or a titanium alloy such as titanium-tantalum,titanium-tantalum-niobium, titanium-palladium or the like. The substratemay be in the form of a plate, perforated plate, bar or net, or in anydesired form.

[0011] The auxiliary anode has an intermediate layer which is providedby a thin film of tantalum or a tantalum alloy. The thin film can beformed by sputtering, ion plating or vacuum evaporation. The tantalumalloy is preferably tantalum-niobium, tantalum-titanium or like alloy.The sputtering processes usable are high-frequency sputtering and d.c.bipolar sputtering. Such a process is preferably magnetron sputtering.The thin film serving as the intermediate layer is preferably 1 to 10μm, more preferably 1 to 7 μm, most preferably 1 to 5 μm, in thickness.When too thin, the thin film will not be formed satisfactory on thesubstrate, whereas excessive thicknesses result in problems such asdifficulty encountered in sputtering. Stated more specifically, ahigh-frequency discharge is effected in an argon gas atmosphere having ahigh vacuum of less than 1×10⁻² torr. The distance between the substrateand the target is suitably determined, for example, from the range of 10to 80 mm. The temperature of the substrate is suitably determined, forexample, from the range of 150 to 230° C. It is further desired to fullyremove the remaining gas. The sputtering operation is conducted for asuitably selected period, for example, from 30 to 200 minutes, whereby atantalum or tantalum alloy thin film of desired thickness is formed onthe substrate.

[0012] A coating layer (catalyst layer) comprising an electrode activesubstance having electrochemical activity is formed on the intermediatelayer thus formed and having no electrode activity. The electrode activesubstance suitable for electrodes involving evolution of oxygencomprises (or contains) a platinum group metal or platinum group metaloxide, or a mixture of a platinum group metal or platinum group metaloxide and an oxide of a valve metal such as titanium, tantalum, niobium,zirconium or the like. Typical examples of electrode active substancescomprises (or contains) a mixture of iridium oxide and tantalum oxide,mixture of iridium oxide and titanium oxide, mixture of iridium oxideand ruthenium oxide, mixture of iridium oxide, ruthenium oxide andtitanium oxide, mixture of ruthenium oxide and titanium oxide, ormixture of ruthenium oxide and tantalum oxide. The electrode activesubstance which is especially preferable is one having high durabilityand comprising (or containing) a mixture of 60 to 95 wt. %, preferably70 to 95 wt. %, of iridium oxide calculated as metallic iridium and 5 to40 wt. %, preferably 5 to 30 wt. %, of tantalum oxide calculated asmetallic tantalum.

[0013] The coating layer comprising an electrode active substance can beformed by a process conventionally used, such as thermal decomposition,electrochemical oxidation or powder sintering. The thermal decompositionprocess is especially desirable. More specifically, a solution of suchmetallic salts is applied to the surface of the intermediate layerseveral times and dried, eventually followed by a heat-treatment at 350to 550° C. The thickness of the coating layer is determined suitably,for example, from the range of 1 to 50 μm.

[0014] The anode, i.e. the main anode may be made, for example, fromlead or a lead alloy. The anode may be an insoluble electrode comprisingan anode material and a coating layer comprising an electrode activesubstance. The electrode active substance for the main anode may besuitably selected from among those mentioned above for the auxiliaryanode.

[0015] The cathode may be made from chromium, titanium or the like.Since the cathode prepared from such a material has a stable oxide filmover the surface, the metallic copper film formed by electrodepositionis unlikely adhere to the underlying cathode surface with a highstrength and can therefore be peeled off from the surface as a thinmetallic film.

[0016] The electrolytic solution and the electrolysis conditions may bethose usually used for forming copper, nickel or like metal foil byelectrodeposition.

[0017] The metal foil production apparatus shown in FIG. 1 has anelectrolysis vessel which is internally provided with a cylindricalcathode 1 rotatable in the direction of an arrow A, and an anode 2having a curved surface opposed to the cathode. In the electrolysisvessel, an electrolytic solution is forced against and supplied to thecathode from a slit 4 in the central lower portion of the anode and isthereby passed through a clearance 5 between the cathode and the anode.Electric current is passed between the cathode 1 and the anode 2 toconduct electrolysis and electrodeposit metallic copper foil ofpredetermined thickness on the surface of the cathode. The copper foilis peeled off and wound up by a winder 6 for continuous production. Theportion of the electrolytic solution flowing through the clearance 5egresses from an outlet 9 of the clearance 5 and returns to the interiorof the vessel.

[0018] The auxiliary anode 7 for adjusting the current density isinstalled as opposed to a position 8 where the unelectrolyzed portion ofthe cathode immersed in the electrolytic solution comes into contactwith the portion of the solution flowing out of the outlet 9 of theclearance 5 between the cathode 1 and the anode 2 and a weak electriccurrent starts to flow. Usually, this position of installation isslightly downstream from the outlet 9 with respect to the direction offlow of the electrolytic solution. The auxiliary anode 7 in the form ofa flat plate is disposed horizontally. The anode 7 is positioned inproximity to the cathode 1, with the solution prevented from reachingthe upper surface of the anode 7. In the case where the cylindricalcathode 1 is 100 mm in diameter and 100 mm in width, with the flatplatelike auxiliary anode 7 measuring 100 mm×5 mm×1.5 mm, the distancebetween the cathode 1 and the auxiliary anode 7 is suitably determined,for example, from the range of 0.3 to 5 mm. The surface of the auxiliaryanode 7 opposed to the cathode 1 has an area sufficient to pass electriccurrent therethrough until particles are uniformly initially depositedon the cathode and grown thereon. The current to be passed from theauxiliary anode 7 to the cathode 1 has such a magnitude as to give acurrent density sufficient to produce nuclei uniformly on the cathodesurface over an area thereof opposed to the auxiliary anode 7. If thecurrent density is increased markedly to a predetermined value by theauxiliary anode 7 in this way at the time a weak current starts to flow,with the cathode 1 coming into contact with the electrolytic solutionwhen electrodeposition is started, over-potential is available which issufficient to produce deposited particles anew over the entire cathodesurface. Consequently, the formation of nuclei with the start of uniformelectrodeposition and growth of particles initially deposited can beeffected over the entire surface without being influenced by the unevenunderlying surface state of the cathode. Thus, the provision of theauxiliary anode obviates uneven formation of nuclei with the start ofelectrodeposition and uneven growth of initially deposited particle thatwould otherwise occur, precluding occurrence of pinholes in the foil dueto an uneven distribution of thicknesses owing to these drawbacks andmaking it possible to produce a thin metal foil product of uniformthickness.

[0019] The current density is adjusted as described above by using asthe auxiliary anode an electrode having a coating layer of electrodeactive substance formed over a conductive metal substrate with anintermediate layer interposed therebetween to thereby eliminate the risein the voltage and local inactivation of the electrode over a longperiod of time, consequently making it possible to produce metal foil ofuniform thickness with good stability.

[0020] The process of the invention is usable also for producing nickelor other metal foil, other than copper foil, by an electrolyticreaction.

EXAMPLES

[0021] The present invention will be described below in greater detailwith reference to examples and comparative examples.

Example 1

[0022] The metal foil production apparatus shown in FIG. 1 has anelectrolysis vessel, which is internally provided with a cylindricalcathode 1 measuring 100 mm in diameter and 100 mm in width and rotatablein the direction of an arrow A, and an anode 2 having a curved surfaceopposed to the cathode. An auxiliary anode 7 in the form of a flat platemeasuring 100 mm×5 mm×1.5 mm for adjusting the initial current densityis disposed horizontally in the electrolysis vessel at one side of thecathode 1 where the cathode in rotation is brought into an electrolyticsolution.

[0023] The auxiliary anode 7 was prepared in the following manner.First, the electrode surface of a titanium plate for the substrate ofthe auxiliary anode was degreased by ultrasonic cleaning, and thetitanium plate was thereafter blasted over both surfaces thereof with#30 alundum under a pressure of 4 kgf/cm² for about 10 minutes. Thetitanium plate thus treated was washed in running water overnight anddried.

[0024] The electrode substrate thus obtained was placed into ahigh-frequency magnetron apparatus. At this time, a tantalum target, 300mm in diameter and 3 mm in thickness, was spaced apart from thesubstrate by a distance of 40 mm, and the chamber was adjusted to aninternal pressure of less than 1×10⁻⁶ torr. Argon gas was introducedinto the chamber to an internal pressure of 1×10⁻² torr. Ahigh-frequency sputtering operation was then conducted at 13.56 MHz forabout 60 minutes. At this time, the high-frequency power applied was 200W (0.3 kV), and the substrate temperature was 170° C. This operationformed a tantalum thin film (intermediate layer) having a thickness ofabout 2 μm and weighing about 30 g/m² on the electrode substrate. Thesurface of the thin film obtained was analyzed by the X-ray diffractionmethod (XRD), which revealed a diffraction pattern of beta-tantalum.

[0025] The following ingredients for an electrode active substancecoating composition was applied to the tantalum thin film on theelectrode substrate. TaCl₅  0.32 g H₂IrCl₆ · 6H₂O  1.00 g 35% HCl  1.0ml n-CH₃ (CH₂) ₃OH 10.0 ml

[0026] The coated substrate was dried at 100° C. for 10 minutes and thenbaked in an electric furnace at 500° C. for 20 minutes. This coatingprocedure to prepare the active substance was repeated five times toform a coating layer comprising iridium oxide serving as an electrodeactive substance (composition ratio by weight of the coating layercalculated as metals: Ir/Ta=7/3).

[0027] With reference to FIG. 1, the electrode thus prepared was used asthe auxiliary anode in the copper foil production apparatus, as opposedto the cathode 1 at a predetermined distance therefrom. The auxiliaryanode was disposed on one side of the rotating cathode 1 where theunelectrolyzed portion thereof was brought into the electrolyticsolution, at a position slightly downstream (with respect to thedirection of flow of the solution) from the outlet 9 of the clearance 5between the cathode 1 and the anode 2, more specifically at a positionwhere the unelectrolyzed portion in the solution came into contact withthe portion of the solution flowing out of the outlet 9 of the clearance5, and a weak current started to flow. The distance between the cathode1 and the auxiliary anode 7 was 2 mm.

[0028] The cathode 1 was made from chromium. The anode 2 was made from alead alloy.

[0029] An aqueous solution containing 100 g/L of sulfuric acid, 250 g/Lof copper sulfate and a glue serving as an additive was prepared as theelectrolytic solution, and the solution was supplied to the electrolysisvessel so as to cause the solution to flow along the cathode surface ata rate of 2 m/sec. The cathode 1 was rotated while passing currentbetween the cathode 1 and the anode 2 at a current density of 120 A/dm²and passing current through the auxiliary anode 7 at 200 A/dm². Theelectrolytic solution was forced out against and supplied to the cathodefrom a slit 4 in the central lower portion of the anode 2 to pass thesolution through the clearance 5 between the cathode 1 and the anode 2for electrolysis and electrodeposit metallic copper foil, 35 μm inthickness, on the cathode surface. The foil was peeled off the cathodeand wound up by a winder 6. The portion of the solution flowing throughthe clearance 5 egressed from the outlet 9 and was returned to theinterior of the electrolysis vessel. The time taken for the electrolysisstarting voltage of the auxiliary anode 7 to increase by 3 V from theinitial value was taken as the electrode life. After conductingelectrolysis for 100 hours, the foil was checked for thickness along thewidth thereof at an interval of 1 cm using a film thickness meter. Table1 shows the result of measurement of foil thickness and the life of theauxiliary anode.

Example 2

[0030] A titanium plate treated in the same manner as in Example 1 waspositioned at a distance of 20 mm from a tantalum target and subjectedto a tantalum sputtering operation. The thin film obtained was checkedby XRD, which revealed a diffraction pattern of alpha-tantalum. Anauxiliary anode was prepared by forming a coating layer of electrodeactive substance on the surface of the thin film by the same procedureas in Example 1. The electrode was tested in the same manner as inExample 1 for electrolytically producing foil. Table 1 shows the resultof measurement of the foil thickness and the life of the auxiliaryanode. The same other procedures as in Example 1 were repeated.

Example 3

[0031] An auxiliary anode having a beta-tantalum intermediate layer wasfabricated in the same manner as in Example 1 with the exception ofusing the following coating composition. TaCl₅  0.18 g H₂IrCl₆ · 6H₂O 1.00 g 35% HCl  1.0 ml n-CH₃ (CH₂) ₃OH 10.0 ml

[0032] The electrode was tested in the same manner as in Example 1 forelectrolytically producing foil. Table 1 shows the result of measurementof the foil thickness and the life of the auxiliary anode.

Example 4

[0033] A titanium plate treated in the same manner as in Example 1 wasmaintained at 50° C. by cooling, positioned at a distance of 40 mm froma tantalum target and subjected to a tantalum sputtering operation inthe same manner as in Example 1. When checked by XRD, the thin filmobtained was found to contain tantalum of amorphous structure. Anauxiliary anode was fabricated by forming a coating layer of electrodeactive substance on the surface of the thin film by the same procedureas in Example 1. The electrode was tested in the same manner as inExample 1 for electrolytically producing foil. Table 1 shows the resultof measurement of the foil thickness and the life of the auxiliaryanode.

Example 5

[0034] A titanium plate treated in the same manner as in Example 1 wasplaced into an ion plating apparatus and subjected to an ion platingoperation using a tantalum vacuum evaporation source to obtain atantalum ion-plating layer (intermediate layer) having a thickness of 5μm. An auxiliary anode was fabricated by forming a coating layer ofelectrode active substance on the surface of this layer by the sameprocedure as in Example 1. The electrode was tested in the same manneras in Example 1 for electrolytically producing foil. Table 1 shows theresult of measurement of the foil thickness and the life of theauxiliary anode.

Comparative Example 1

[0035] The same electrode coating composition as used in Example 1 wasapplied directly to the surface of a titanium plate treated in the samemanner as in Example 1 to similarly form a coating layer wherein iridiumoxide served as an electrode active substance and to fabricate anauxiliary anode. The electrode was tested in the same manner as inExample 1 for electrolytically producing foil. Table 1 shows the resultof measurement of the foil thickness and the life of the auxiliaryanode.

Example 6

[0036] The same auxiliary anode as used in Example 1 was tested forelectrolytically producing foil by the same procedure as in Example 1except that a pulse current was used as the current. The pulseelectrolysis conditions were a current density of 200 A/dm², pulseelectrolysis time of 10 milliseconds and cessation time of 10milliseconds. The time taken for the electrolysis starting voltage ofthe auxiliary anode to increase by 3 V from the initial value was takenas the electrode life. Table 2 shows the life of the auxiliary anode.

Comparative Example 2

[0037] The same auxiliary anode as used in Comparative Example 1 wasused under the same conditions as in Example 6 for a pulse electrolysistest. Table 2 shows the result. TABLE 1 Thickness of Ta Uniformity ofintermediate foil thickness Electrode life layer μm 100 hrs later(hours) Example 1 2 Good 1150 Example 2 2 Good 1040 Example 3 2 Good1190 Example 4 2 Good 1100 Example 5 5 Good 970 Comp. Ex. 1 0 Poor 460

[0038] TABLE 2 Thickness of Ta intermediate layer Electrode life μm(hours) Example 6 2 870 Comp. Ex. 2 0 240

[0039] The foregoing results of Examples and Comparative Examples revealthat the use of the auxiliary anode having an intermediate layer oftantalum results in an electrode life twice as long as that provided byan auxiliary anode having no intermediate layer, further producing foilof uniform thickness and satisfactory quality without permitting localseparation of the coating layer, i.e., the catalyst layer.

[0040] The auxiliary anode for use in the present invention comprises aconductive metal substrate of titanium or an alloy thereof, and acoating layer of electrode active substance formed over the substrate,with an intermediate layer of tantalum or an alloy thereof interposedtherebetween. The intermediate layer prevents electrolytic oxidation ofthe substrate of titanium or alloy thereof and itself has corrosionresistance, resistance to electrolytic oxidation and satisfactoryelectrical conductivity. These characteristics are useful for pulseelectrolysis. Furthermore, the coating layer comprising an electrodeactive substance and formed on the intermediate layer retains goodadhesion to the intermediate layer, has high catalytic activity andremains highly durable against local inactivation over a prolongedperiod of time. The auxiliary anode having the intermediate layer isexceedingly superior in these characteristics to the auxiliary anodehaving no intermediate layer.

What is claimed is:
 1. In producing metal foil by passing an electriccurrent between a cylindrical cathode immersed in an electrolyticsolution and an anode opposed to the cathode, continuouslyelectrodepositing a metal layer on a surface of the cathode whilerotating the cathode and thereafter peeling the metal layer off, aprocess for producing metal foil characterized in that an auxiliaryanode capable of adjusting the current density when electrodeposition isstarted is disposed at one side of the cathode where an unelectrolyzedportion thereof is brought into the electrolytic solution, at a positiondownstream from the anode with respect to the direction of flow of theelectrolytic solution, the auxiliary anode being an electrode having acoating layer comprising an electrode active substance containing aplatinum group metal or a platinum group metal oxide, or a mixture of anoxide of a valve metal and a platinum group metal or a platinum groupmetal oxide and formed over an electrically conductive metal substratecomprising titanium or a titanium alloy, with an intermediate layer oftantalum or a tantalum alloy formed between the coating layer and thesubstrate.
 2. A process for producing metal foil according to claim 1wherein the metal foil is copper foil.
 3. A process for producing metalfoil according to claim 1 wherein the titanium alloy is an alloyselected from the group consisting of titanium-tantalum,titanium-tantalum-niobium and titanium-palladium, or a combination of atleast two of these alloys.
 4. A process for producing metal foilaccording to claim 1 wherein the intermediate layer is formed bysputtering, ion plating or vacuum evaporation.
 5. A process forproducing metal foil according to claim 1 wherein the tantalum alloy isa tantalum-niobium alloy and/or a tantalum-titanium alloy.
 6. A processfor producing metal foil according to claim 1 wherein the intermediatelayer is 1 to 10 μm in thickness.
 7. A process for producing metal foilaccording to claim 1 wherein the valve metal is selected from the groupconsisting of titanium, tantalum, niobium and zirconium, or is acombination of at least two of these metals.
 8. A process for producingmetal foil according to claim 1 wherein the electrode active substanceis a mixture selected from the group consisting of a mixture of iridiumoxide and tantalum oxide, mixture of iridium oxide and titanium oxide,mixture of iridium oxide and ruthenium oxide, mixture of iridium oxide,ruthenium oxide and titanium oxide, mixture of ruthenium oxide andtitanium oxide, and mixture of ruthenium oxide and tantalum oxide, or acombination of at least two of these mixtures.
 9. A process forproducing metal foil according to claim 1 wherein the electrode activesubstance contains a mixture of 60 to 95 wt. % of iridium oxidecalculated as metallic iridium and 5 to 40 wt. % of tantalum oxidecalculated as metallic tantalum.
 10. A process for producing metal foilaccording to claim 1 wherein the electrode active substance contains amixture of 70 to 95 wt. % of iridium oxide calculated as metalliciridium and 5 to 30 wt. % of tantalum oxide calculated as metallictantalum.
 11. A process for producing metal foil according to claim 1wherein the coating layer is formed by thermal decomposition,electrochemical oxidation or powder sintering.
 12. A process forproducing metal foil according to claim 1 wherein the coating layer is 1to 50 μm in thickness.